In common rail fuel systems for compression ignition internal combustion engines, fuel is pressurized by means of a high-pressure fuel pump, which is supplied with fuel from a fuel tank by a low-pressure transfer pump. Typically, the high-pressure fuel pump comprises a main pump housing supporting multiple pump elements. Each pump element includes a plunger, which is driven in a reciprocating motion by an engine-driven camshaft to generate high fuel pressure. Fuel at high pressure is then stored in a common fuel rail for delivery to fuel injectors.
Typically, a single inlet metering valve is used to meter the fuel entering all of the pump elements. Fuel in the pump elements becomes pressurized during a pumping stroke of the associated plunger. The provision of the inlet metering valve means that, throughout the operational range of the engine, the pumping duty of the high-pressure fuel pump is distributed equally between the pump elements, regardless of whether or not the pump elements are being operated at less than their maximum pumping capacity. Accordingly, the frequency with which each pump element is required to perform a pumping stroke is a maximum.
The Applicant's co-pending EP patent application 09157959.9 describes an alternative fuel pump in which, rather than having a single inlet metering valve across all pump elements, each pump element is provided with its own dedicated metering valve. The plunger of each pump element is driven by an associated engine-driven cam having one or more cam lobes. The control valve of each pump element is operable during a pumping window between bottom-dead-centre and top-dead-centre, corresponding to the rising flank of the relevant cam lobe, to control the quantity of fuel delivered to the rail. The duration of each pumping event within the pumping window determines the quantity of fuel delivered by the pump element into the common rail. In order to achieve the required duration of pumping, the valve must be actuated at the correct position in engine revolution relative to the cam during the pumping window. To achieve full pump capacity for a pump element, the metering valve of that element is actuated over the full pumping window, whereas for zero demand the valve is not actuated over any of the pumping window.
The invention in EP 09157959.9 provides the advantage that the pumping duty of at least one of the pump elements (or at least one of the cam lobes associated with a pump element) can be removed easily by not operating the metering valve associated with that specific pump element, meaning it is not exposed to a pressurising phase of the pumping stroke. The frequency with which that pump element is subject to a pumping stroke is therefore reduced, together with the possibility of fatigue failure. Furthermore, it has been recognised that due to clearances between components of the pump elements, the pump elements are subject to high-pressure fuel leakages during the pumping stroke. The high-pressure fuel leakages represent a reduction in pump efficiency as the pressurized fuel is not entirely displaced to the common fuel rail. The invention in EP patent application 09157959.9 overcomes this problem.
Another desirable feature of such common rail fuel pumps is that rail pressure is controlled and maintained accurately so as to maintain injection pressure. It is an object of the present invention to provide a method of controlling rail pressure in a common rail fuel pump of the aforementioned type in which this object is achieved.